1. Field of the Invention
The invention relates to a system for detecting and positioning objects or objectives, both above and under sea-level, the electric conductivity of which differs from the surrounding medium thereof by measuring variations in an electric field generated into the medium. The examples given in the description below illustrate applications of systems according to the invention mainly for a purpose of surveillance of fairways and bodies of water. More precisely the invention concerns a method and assembly for detecting objects or objectives, for example vessels, in an electrically conductive medium, for example sea, by measuring variations, signs, indications, disturbances or the like caused by the object or objective, especially for surveying water fairways and/or extent, in the following referred to as areas.
2. Description of Related Art
Electric conductivity measurements are previously known and commonly used for geophysical surveys, e.g. in prospecting of ore, oil and gas deposits, mainly to supplement other surveying methods. (See e.g. Geophysics, vol. 49, p. 439).
The use of electromagnetic and sonic waves for detecting ships is known in the prior art. In case of radio waves being used for radar measurements, remarkable problems and drawbacks are involved. For example, by detecting the measuring signals one may easily perceive from an objective that there exists a surveillance monitoring or measurements. However, it is difficult to detect objectives made of non-conducting material. Radars can be located without difficulty, and further the use thereof normally requires the presence of an operator.
Underwater acoustic detecting apparatuses, sounding systems and hydrophones, are known methods for detecting submarines. Therein the propagation of sound in water is made use of. The propagation of sonic waves in water is not linear in all circumstances; the waves can be reflected, e.g., from temperature difference layers, the sound becoming distorted, and the water may contain zones where it is not possible to make acoustic observations. On the other hand, it is difficult to make acoustic detecting observations if the objective vessel lies unmoved at the bottom and in the case of interference noises coming from the area under surveillance. The use of acoustic detecting apparatuses requires sensitive apparatuses and skilled operating personnel.
For detecting submarines also long radio waves can be used and, e.g., infrared light which propagates to a longer distance than an ordinary light wave, especially in muddy water. However, electromagnetic waves normally attenuate rapidly in water and, for this reason, observation ranges remain short. Through a patent document it seems to have become known that electric fields generated by strokes of lightning may be used for detecting submerged objects or objectives at a long distance.
Submarines can also be detected by measuring variations they have caused in magnetic fields or gravitation fields. These measuring methods, however, are often expensive, the computing algorithms complicated and the measurement range limited.
Ore and oil prospecting methods based on modelling and measuring of the resistivity of soil have become known, but their use has remained insignificant as the resistivity of soil changes pursuant to rainfalls and seasons and as the topography of the soil and other, deeper existing layers of earth complicate the making of an accurate and practicable model of the electric conductivity of normal soil and, in addition, it is difficult to measure the potential of a subsurface field. Therefore, the accuracy of the method in measuring the soil is high enough only for detecting sufficiently great anomalies, and more accurate surveys are made by other methods. However, in these geophysical surveying methods mathematical models and computing algorithms have been developed that are practicable in some applications of the method according to the invention.
No publications have been found regarding the use of the resistivity measuring method for surveillance of sea areas, which may be due to the fact that the resistivity of seawater normally is very low and changes in resistivity are difficult to detect and, consequently, there has been no trust in the practicability of the method.